Risk Assessment Of A Landfill Using Soil Gas Survey

The soil gas survey technique was applied at a codisposal facility to define the direction<br>and extent of a contaminated soil gas plume. A typical soil gas probe assembly was used to<br>collect soil gas samples from approximately 2.5 feet below the ground surface. Photovac lOS50<br>portable gas chromatographs (GC) were used for field analysis of approximately 120 samples<br>during three sampling events. In addition, air samples were taken from the landfill surface. Each<br>sample was analyzed for five target volatile organic compounds (VOCs): vinyl chloride 0/C),<br>1,l -dichloroethene (1,l -DCE), trichloroethene (TCE), tetrachloroethene (PCE), and benzene. The<br>main criterion for selecting these compounds was their level of toxicity.<br>The similarity of the soil gas contaminants to those measured at the landfill surface (flux<br>box) indicates that landfill air emissions may be a primary source for the compounds identified in<br>the soil gas. Potentially, the presence of a polyvinyl chloride (PVC) cap over the municipal/<br>industrial landfill could be forcing landfill gas out of the perimeter of the landfill into the adjacent<br>soil. Large amounts of landfill-generated methane may create a significant pressure gradient in the<br>landfill and cause a vapor phase plume to migrate in the direction of lower pressure off the landfill.<br>Soil gas is further considered to be a source of contamination for ambient air above ground.<br>An advective transport model was used to predict the concentrations of contaminants in air.<br>Ambient air monitoring results showed a good correlation with the values predicted by the models.<br>Traditionally, the soil gas survey technique is used to define the direction and extent of<br>contamination in soil and groundwater. In the present study, this technique was used innovatively<br>to define the soil gas plume and then for risk assessment purposes via the air-vapor pathway.